Mobile communication system, communication control method, and base station and mobile station to be employed in the same

ABSTRACT

In the communication system performing transmission with generating the vacant period in the slotted mode during data transmission in order to perform various process, such as different frequency carrier quality measurement or so forth, the control signal for maintaining a communication quality is inserted even in the vacant period. Therefor, a link quality may not be degraded even if the vacant period is long.  
     Namely, when the pilot signal is used as the control signal, even in the vacant period, measurement of the link quality of the forward link with the pilot signal is performed in the mobile station to report the result of measurement to the base station. Therefore, transmission power control of the forward link can be performed according to the report in the base station. Therefore, degradation of the forward link can be eliminated.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to a mobile communication system, a communication control method, and a base station and a mobile station to be employed in the mobile communication system. More particularly, the invention relates to a mobile communication system, in which a vacant time containing no communication data is provided in one or more of communication frames to be transmitted from the base station to the mobile station and a process, such as quality measurement of radio link of a different frequency carrier, for example, is performed during the vacant period at the mobile station.

[0003] 2. Description of the Related Art

[0004] In the mobile communication system, wide service area is realized by providing unit communication service regions called as cells and establishing a service area as aggregate of the cells. The cell is defined as a range to be covered by one base station. On the other hand, since each base station has to cover a large number of user signals and so forth, a plurality of frequency carriers useful for communication are provided. For example, in an arrangement of frequency shown in FIG. 8, frequency carriers 401 and 402 can be used simultaneously by the same base station.

[0005] When the mobile station during communication moves within the service area, transition is caused from the current cell and a new and adjacent cell to perform switching of radio links in order to switch the base stations for establishing connection. For example, in FIG. 9, there is a case to switch communication link from a link 601 to a link 611. Transitional process to be caused upon switching of the radio links to connect is referred to as “hand over”. Switching of the radio links will be caused even when the mobile station does not move. For example, when a quality of the link in connection is low, when a communication speed is desired to be varied but variation of the communication speed cannot be achieved by the link in current connection, or so forth, switching of links to connect can be caused between the radio links of the same base station having a plurality of carriers (between the links 601 and 603 in FIG. 9) or between the radio links of a plurality of the base stations in the case where a plurality of base stations have cells simultaneously covering a point where the mobile station presents.

[0006] On the other hand, when a frequency of the carrier used in communication in the radio link before switching and a frequency of the carrier used in communication in the radio link after switching, the hand over is particularly referred to as “different frequency hand over”. In order to realize different frequency hand over, it becomes necessary to perform measurement of link quality in order to select a frequency carrier having high radio link quality as object for switching.

[0007] Conventional, in the case where own station has a period not performing transmission and reception, such as time division multiple access (TDMA), utilizing the vacant slot period, there is a method, in which a radio frequency is switched from the radio frequency in current communication to a frequency of measurement object for using quality measurement of the radio link of the different frequency carrier. In this method, since communication and measurement of the different frequency carrier are not performed simultaneously, it can be adapted by switching of the radio frequency and it is not necessary to separately provide a radio machine for measuring the different frequency carrier.

[0008] On the other hand, in a system based on continuous transmission, such as frequency-division multiple access (FDMA) or code division multiple access (CDMA), there is a method to use two or more of plurality of radio machines, and to use the radio machine not used in communication for quality measurement of the radio link of other frequency carrier. As a method for not using two or more radio machines in CDMA, there has been proposed a Slotted mode (M. Gustafsson et al., “Compressed Mode Techniques for Inter-Frequency Measurements in a Wide-band DS-CDMA System”, Proceedings of the 8^(th) PIMRC, September, 1997: First Prior Art). This is a system to compress a signal in time direction and to provide a vacant time having no data signal with maintaining a diffusing band unchanged by lowering spreading ratio over a plurality of time or elevating coding ratio of error correction coding, namely performing so-called puncturing.

[0009]FIG. 10 is an illustration showing an example of the case where the vacant period is provided. In FIG. 10, S1 to Sk+1 represent time slots. Referring to FIG. 10, there is shown one example, in which a signal transmission speed is increased to be double in comparison with those in periods T1 and T3 for compressing the signal into half in a time axis by lowering a spreading ratio in a period T2 into half. In a vacant period T4 thus obtained, a radio frequency is switched from a frequency carrier currently used for communication to another different frequency carrier to measure quality of the carrier.

[0010] On the other hand, there is another technology, in which a channel prediction (detection of amplitude information and phase information) is performed precisely by using a plurality of pilot blocks inserted in a transmission signal and a signal to noise power ratio is lowered for obtaining a necessary reception quality (bit error ratio or so forth) in synchronous or coherent detection (“Cannel Estimation Scheme Using the Plurality Pilot Blocks for DS-CDMA Mobile station”, Shingaku Giho, The institute of Electronics. Information and Communication Engineers, Japan, August, 1996, pp 45 to 50).

[0011] On the other hand, there is a further method for controlling a transmission power in a reverse link in the mobile station by performing transmission with inserting the pilot signal in an information system string from the mobile station, measuring a quality (SIR: signal power to interference power ratio) of a reverse link by receiving the pilot signal in the base station, comparing the SIR with a target value and reporting a result of comparison to the mobile station.

[0012] Furthermore, the pilot signal is also used for controlling the transmission power of a forward link. Namely, in the mobile station, the pilot signal transmitted from the base station is received for measuring the quality of the forward link, the measured quality, i.e. SIR, is compared with the target value to report the comparison result to the base station. Then, according to the result of comparison, the transmission power in the forward link is controlled in the base station.

[0013] However, in case of the slotted mode set forth above, the information for controlling the transmission power of the reverse link cannot be transmitted during vacant period. Therefore, problem is encountered in degradation of characteristics of the reverse link. This problem will be discussed with reference to FIG. 9. It is assumed that the mobile station 620 connected to the base station 600 is performing communication using the link 601 as the forward link and the link 602 as the reverse link.

[0014] Structure of the transmission signal on the link 601 is illustrated in FIGS. 11A to 11E, for example. As shown in FIG. 11A, there is a super frame consisted of m in number of frames Fl to Fm. Each of the frames Fi is segmented into time slots S1 to Sn with a given interval as shown in FIG. 11B. Respective of the time slots Sj are in three kinds of forms as illustrated in FIGS. 11C to 11E. In FIG. 11C, a-pilot signal PL, a transmission power control signal TPC, a transmission rate information RI and transmission data D1 are arranged in sequential order from the leading end of the time slot.

[0015] In FIG. 11D, a pilot signal PL, a transmission rate information RI, transmission data D1, a transmission power control signal TPC and transmission data D2 are arranged in sequential order from the leading end of the time slot. In FIG. 11E, a transmission rate information RI, transmission data D1, a transmission power control signal TPC, transmission data D2 and a pilot signal PL are arranged in sequential order from the leading end of the time solt.

[0016] The pilot signal PL is a signal for synchronous detection and quality measurement as set forth above. The transmission power control signal TPC is a transmission power control information in the reverse link (e.g. reverse link 602). At this time, when the link 601 enters into a different frequency carrier measurement mode and thus enters into the vacant period T4 of FIG. 10, the transmission power control information of the reverse link 602 becomes not transmitted by the link 601, the transmission power of the mobile station 620 is offset from an appropriate level to cause degradation of the characteristics of the reverse link 602. This degradation becomes more significant at longer vacant period T4.

[0017] On the other hand, when reception using a plurality of pilot symbols is to be performed in the mobile station, reception using the former and later pilot symbols becomes impossible due to presence of the vacant period T4 to cause degradation of the reception quality. On the other hand, when transmission of the pilot signal from the base station is not effected, SIR measurement in the mobile station is not performed. As a result, the transmission power control of the forward link in the base station becomes impossible.

SUMMARY OF THE INVENTION

[0018] It is an object of the present invention to provide a mobile communication system, a communication control method, and a base station and a mobile, in which degradation of a transmission power control characteristics of a reverse link in a vacant period in a slotted mode, can be restricted to be small.

[0019] Another object of the present invention is to provide a mobile communication system, a communication control method, and a base station and a mobile, in which degradation of reception quality due to impossibility of reception using former and later pilot symbols upon reception using a plurality of pilot symbols.

[0020] A further object of the present invention is to invention to provide a mobile communication system, a communication control method, and a base station and a mobile which can prevent occurrence that the transmission power control in forward link due to impossibility of transmission of a pilot signal from the base station, becomes impossible.

[0021] According to the first aspect of the present invention, a mobile communication system comprising:

[0022] transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames, and inserting a first control signal for maintaining a communication quality in the vacant period.

[0023] According to the second aspect according to the present invention, a mobile communication system comprising:

[0024] transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames from a timing immediately after a second control signal for maintaining communication quality.

[0025] According to the third aspect according to the present invention, a mobile communication system comprising:

[0026] transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of the vacant period,

[0027] each of the second and third control signals being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.

[0028] According to the fourth aspect according to the present invention, a mobile communication system comprising:

[0029] transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of the vacant period,

[0030] the second control signal being a transmission power control for a reverse link and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.

[0031] According to the fifth aspect according to the present invention, a communication control method mobile communication system comprising:

[0032] a step of providing a vacant period, in which no communication data is present, in one or more of communication frames, and

[0033] a step of inserting a first control signal for maintaining a communication quality in the vacant period, for transmission.

[0034] According to the sixth aspect according to the present invention, a communication control method comprising:

[0035] step of transmitting a second control signal for maintaining a communication quality in one or more of communication frames; and

[0036] step of providing a vacant period, in which no communication data is present, at a timing immediately after the second control signal.

[0037] According to the seventh aspect according to the present invention, a communication control method comprising:

[0038] step of providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of the vacant period,

[0039] the second and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.

[0040] According to the eighth aspect according to the present invention, a communication control method comprising:

[0041] step of providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of the vacant period,

[0042] the second control signal being a transmission power control for a reverse link and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.

[0043] According to the ninth aspect according to the present invention, a base station in a mobile communication system, comprising:

[0044] transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames, and inserting a first control signal for maintaining a communication quality in the vacant period.

[0045] According to the tenth aspect according to the present invention, a base station in a mobile communication system, comprising:

[0046] transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames from a timing immediately after a second control signal for maintaining communication quality.

[0047] According to the eleventh aspect according to the present invention, a base station in a mobile communication system, comprising:

[0048] transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of the vacant period,

[0049] the second and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.

[0050] According to the twelfth aspect according to the present invention, a base station in a mobile communication system, comprising:

[0051] transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of the vacant period,

[0052] the second control signal being a transmission power control for a reverse link and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.

[0053] According to the thirteenth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0054] quality measuring means for receiving a first control signal for maintaining a communication quality transmitted in a vacant period in which no communication data is present, in one or more of communication frames and measuring a reception quality on the basis of the first control signal; and

[0055] transmitting means for generating and transmitting a transmission power control information for a forward link according to the reception quality.

[0056] According to the fourteenth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0057] demodulation means for receiving a first control signal for maintaining a communication quality transmitted in a vacant period in which no communication data is present, in one or more of communication frames and demodulating a communication data using the first control signal.

[0058] According to the fifteenth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0059] means for receiving a first control signal for maintaining a communication quality transmitted in a vacant period in which no communication data is present, in one or more of communication frames and controlling a transmission power in a reverse link according to the first control signal.

[0060] According to the sixteenth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0061] quality measuring means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and measuring a reception quality on the basis of the second control signal; and

[0062] transmitting means for generating and transmitting a transmission power control information for a forward link according to the reception quality.

[0063] According to the seventeenth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0064] demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and demodulating a communication data using the second control signal.

[0065] According to the eighteenth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0066] means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and controlling a transmission power in a reverse link on the basis of the second control signal.

[0067] According to the nineteenth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0068] quality measuring means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after the vacant period, and measuring a reception quality on the basis of the second or third control signal; and

[0069] transmitting means for generating and transmitting a transmission power control information for a forward link according to the reception quality.

[0070] According to the twentieth aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0071] demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after the vacant period, and demodulating a communication data using the second or third control signal.

[0072] According to the twenty-first aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0073] quality measuring means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after the vacant period, and measuring a reception quality on the basis of the third control signal;

[0074] transmitting means for generating and transmitting a transmission power control information for a forward link according to the reception quality; and

[0075] means for controlling a transmission power in a reverse link on the basis of the second control signal.

[0076] According to the twenty-second aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0077] demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after the vacant period, and demodulating a communication data using the third control signal; and

[0078] means for controlling a transmission power in a reverse link on the basis of the second control signal.

[0079] According to the twenty-third aspect according to the present invention, a mobile station in a mobile communication system, comprising:

[0080] demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after the vacant period, and demodulating a communication data using the third control signal;

[0081] quality measuring means for measuring a reception quality on the basis of the third control signal;

[0082] transmitting means for generating and transmitting a transmission power control information in a forward link according to the reception quality; and

[0083] means for controlling a transmission power in a reverse link on the basis of the second control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0084] The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to be limitative to the invention, but are for explanation and understanding only.

[0085] In the drawings:

[0086]FIG. 1 is a block diagram of the preferred embodiment of a base station according to the present invention;

[0087]FIG. 2 is a flowchart showing an operation of the preferred embodiment of the base station of FIG. 1;

[0088]FIG. 3 is an illustration showing one example of insertion of a control signal in the preferred embodiment of the present invention;

[0089]FIG. 4 is an illustration showing another example of insertion of a control signal in the preferred embodiment of the present invention;

[0090]FIG. 5 is an illustration showing a further example of insertion of a control signal in the preferred embodiment of the present invention;

[0091]FIG. 6 is a block diagram of the preferred embodiment of a mobile station according to the present invention;

[0092]FIG. 7 is a flowchart showing operation of the preferred embodiment of the mobile station of FIG. 6;

[0093]FIG. 8 is an illustration showing an example of arrangement of a frequency carrier;

[0094]FIG. 9 is an illustration for explaining links between a base station and a mobile station; and

[0095]FIG. 10 is an illustration showing an example of a vacant period in the conventional slotted mode; and

[0096]FIGS. 11A to 11E are illustrations showing examples of formats of a transmission signal of a forward link from the base station.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0097] The present invention will be discussed hereinafter in detail in terms of the preferred embodiment of the present invention with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. It will be obvious, however, to those skilled in the art that the present invention may be practiced without these specific details. In other instance, well-known structures are not shown in detail in order to avoid unnecessarily obscure the present invention.

[0098]FIG. 1 is a block diagram of the preferred embodiment of a base station according to the present invention. An input transmission data is input to an error correction coding portion 1 for effecting error correction coding. The error correction codes thus generated are supplied to a segmenting portion 2. The segmenting portion 2 is responsive to the input error correction codes to generate slots as generated segments. In each slot, a pilot control signal PL, a transmission power control signal TPC, a transmission rate information RI and so forth, respectively shown in FIGS. 11C to 11E, are inserted at respectively predetermined positions. An output of the segmenting portion 2 is modulated by a modulating portion 3 and is supplied to an antenna 7 via a spreading portion 4, a power amplifier 5 and a transmission/reception common circuit 6, and transmitted therefrom.

[0099] A reception signal from a mobile station which will be discussed later, is supplied to a despreading portion 8 through the antenna 7, and the transmission/reception common circuit 6. Then, the reception signal is demodulated in a demodulating portion 9. A pilot signal extracting portion 10 extracts the pilot signal among the control signals from the demodulated output of the demodulating portion 9. Then, SIR measurement of the reverse link is performed by a SIR measuring portion 11. In a TPC signal generating portion 12, the measured SIR value and a targeted value are compared for generating a transmission power control information TPC for the reverse link on the basis of the result of comparison. The transmission power control information TPC is transmitted to a control signal inserting portion 13 to be inserted in respective segments together other control information, such as the pilot signal. On the other hand, by a TPC signal extracting portion 14, a transmission power control information TPC for a forward link is extracted from the output of the demodulator 9. On the basis of this transmission power control information TPC, gain control of the power amplifier 5 is performed.

[0100] Here, as a method for generating a vacant period T4 shown in FIG. 10, there are a method by puncturing and a method by lowering a spreading ratio, as set forth above. The former method is a system for thinning data of sixteen time slots into data of fifteen time slots for example, by the error correction coding portion 1. By this, the vacant period for one slot can be produced. The later method of lowering the spreading rate is a system for compressing the signal into a half in a time axis link by increasing a transmission speed to be double, by lowering the spreading ratio to be one-half by segmenting the sixteen time slots into eight segments. This method can produce relatively long vacant period. Utilizing this vacant period, quality measurement of different frequency carrier or various other processes are performed in the mobile station. In the present invention, even in the vacant period, insertion of the control signal is performed by the control signal inserting portion 13.

[0101]FIG. 2 is a flowchart generally showing operation of the base station shown in FIG. 1. Referring to FIG. 2, at first, in response to error correction coding of the transmission data (step S1), segmenting is performed (step S2). At this time, vacant periods are produced with a given interval (step S3). Thereafter, insertion of the control signals to respective segments is performed during a normal data transmission period (steps S4 and S10). The control signals are inserted in the vacant period, immediately before the vacant period or immediately after the vacant period (steps S4 and S5). Then, transmission of signal is performed with performing the transmission power control (step S6).

[0102] On the other hand, the control signals are extracted from a received data (step S7), and SIR measurement is performed with the pilot signals in the control signals (step S8). The SIR value as a result of measurement is compared with the target value for generating the transmission power control information TPC for the reverse link on the basis of the result of comparison (step S9). Together with another control signal, such as the pilot signal, the transmission power control information TPC is inserted in respective of the segments and the vacant periods, respectively (steps S5 and S10). On the other hand, according to the transmission power control information TPC for the forward link extracted at step S7, the transmission power control is performed (Step S6).

[0103] Various embodiments inserting the control signals into the vacant period are illustrated in FIGS. 3 to 5. The embodiment of FIG. 3 is particularly effective when the vacant period T4 is relatively long, which corresponds to the case where the vacant period is certainly provided by varying the spreading ratio in the slotted mode. When the vacant period is relatively long, instead of providing one series of vacant period, and if the process to be performed during the vacant period is quality measurement of the different frequency carrier, the vacant period is divided into sub-blocks T41, T42 of the period corresponding to minimum necessary period. Then, during an interval between the sub-blocks T41, T42, transmission of the control signal 101 is performed.

[0104] In FIG. 3, there is shown a case where the control signal transmission in the vacant period T4 is only once. However, it is obvious that transmission of the control signal is performed for a plurality of times. On the other hand, at this time, a transmission internal between a plurality of control signals to be inserted in the vacant period T4 may be set to be longer than a transmission interval of the same kind of control signals in the normal transmission mode.

[0105] In the embodiment of FIG. 4, the vacant period T4 is relatively short and is applicable for the case where quality measurement of the different frequency carrier is difficult to be performed during a period by dividing the vacant period into sub-blocks, for example. Mainly, obtaining the vacant period by puncturing in the slotted mode is the case. In the shown embodiment, before entering into the vacant period T4, transmission of the control signal 101 immediately after information signal transmission in the quality measurement mode of the different frequency carrier is performed. Thereafter, the process enters into that in the vacant period. Since the transmission power control for the reverse link is performed in response to the control signal, characteristics of the reverse link can be improved in comparison with the case where the process in the vacant period is entered immediately after information signal transmission as in the prior art.

[0106] The shown embodiment, as in the case of FIGS. 11C, 11D and 11E, is effective in a form of time slot where a part or all of control signals are present in the vicinity of the leading end of the slot where the vacant period T4 is started. The shown embodiment is the case where the vacant period T4 is short. However, it should be obvious that even when the vacant period is long, as shown in FIG. 4, the quality measurement of the different frequency carrier is initiated after transmission up to the control signal before entering into the vacant period without dividing the vacant period into the sub-blocks, as shown in FIG. 4.

[0107] In the embodiment shown in FIG. 5, the control signal 101 is inserted immediately after the vacant period T4. The shown embodiment is effect in case of the time slot form, in which a part of or all of control signals are present at the trailing end of the time slot, at which the vacant period T4 ends, as in the case shown in FIG. 1E. It is, of course, possible in the shown embodiment, to insert the control signal at a given interval. On the other hand, it is also possible to perform control signal transmission even at immediately before the vacant period as shown in FIG. 4 and terminate transmission of the communication data for the mobile station which performs reception of the different frequency carrier, after the control signal transmission.

[0108]FIG. 6 is a block diagram of the preferred embodiment of a mobile station according to the present invention. Referring to FIG. 6, a reception signal from an antenna 21 is input to a despreading portion 23 via a transmission and reception common circuit 22 to perform despreading process. An output of the despreading portion 23 is, demodulated by a demodulating portion 24 for outputting a demodulated output. On the other hand, an input transmission data is segmented by a segmenting portion 25 and input to a modulating portion 26. An output of the modulating portion 26 is supplied to the transmission and reception common circuit 22 via a spreading circuit 27 and a power amplifier 28 and transmitted from the antenna 21.

[0109] In a pilot signal extracting portion 29, a pilot signal is extracted from a signal output from the demodulating portion 24 and transmitted to a SIR measuring portion 30. In the STR measuring portion 30, SIR in the forward link is measured. In a TPC signal generating portion 31, the SIR as measured value and the target value are compared. A comparison result is inserted in each segment as the transmission power control signal TPC for forward link by the control signal inserting portion 32 together with the pilot signal as other control signals.

[0110] On the other hand, in a TPC signal extracting portion 33, a transmission power control signal TPC for reverse link is extracted from the signal of the demodulating portion 24. On the basis of the transmission power control signal TPC, a gain control of the power amplifier 28 is performed.

[0111]FIG. 7 is a schematic block diagram showing operation of the mobile station shown in FIG. 6. Referring to FIG. 7, segmenting of the transmission data is performed (step S21). Then, the control signal is inserted to each segment (step S22). Then, transmission is performed with performing transmission power control (step S23).

[0112] On the other hand, extraction of the control signal from the received data is performed (step S24). By the pilot signal in the control signal, SIR measurement is performed (step S25). Comparison of the SIR value and the target value is performed. Then, on the basis of the result of comparison, the transmission power control signal TPC for forward link is generated (step S26). The generated transmission power control signal TPC is inserted into the segment at step S22. On the other hand, according to the transmission control signal TPC for the reverse link in the control signal extracted from the reception data, transmission power control is performed (step S23). Thus, transmission in reverse link is performed.

[0113] In the foregoing embodiment, discussion has been given that the pilot signal PL and the transmission power control information TPC are respectively contained as control signals for the reverse link and the forward link, In this case, control of the transmission power in both of the reverse link and the forward link can be performed. Particularly, even in the vacant period T4, by employing a construction, in which the pilot signal PL and the transmission power control information TPC are included as the control signals 101, transmission power control for the reverse link can be performed at higher frequency in the corresponding extent in the mobile station. Therefore, degradation of transmission power characteristics in the reverse link can be successfully alleviated, particularly.

[0114] On the other hand, as another embodiment, it is possible that only pilot signal PL is included in the control signal 101 in the vacant period T4. For example, as shown in FIGS. 11C and 11D, when the pilot signal PL is arranged at the leading end of the time slot, it is possible to terminate transmission of the communication data to the mobile station which performs reception of the different frequency carrier after transmission up to the pilot signal PL as the control signal immediately after transmission of the information signal, as shown in FIG. 4.

[0115] On the other hand, when the pilot signal PL is arranged at the trailing end of the time slot as shown in FIG. 1E, the pilot signal PL as the control signal immediately after the vacant period is inserted, and transmission is initiated from the inserted pilot signal, as shown in FIG. 5. In these case, the mobile station may demodulate the reception data immediately before the vacant period and the reception data immediately after the vacant period using the former and later pilot signals. Therefore, reception quality in the forward link can be improved.

[0116] On the other hand, the pilot signal PL transmitted from the base station is received by the mobile station. Then, with the pilot signal PL, SIR measurement is performed. The result of measurement is reported to the base station to enable control of the transmission power of the forward link in the base station. As a result, link quality in the forward link can be improved. Then, by transmitting the pilot signal immediately before termination of transmission (immediately before starting of the vacant period), SIR measurement is performed using the pilot signal in the mobile station. Thus, one greater number of transmission power control instruction can be transmitted to the base station to improve the transmission power control of the forward link.

[0117] On the other hand, it is possible to transmit the control signals at both of immediately before and immediately after vacant period. For example, in case of the time slot structure shown in FIG. 1E, the control signal inserted immediately before the vacant period can be the transmission power control information TPC and the control signal inserted immediately after the vacant period can be the pilot signal PL. In this case, the base station terminates transmission of the communication data to the mobile station which performs reception of the different frequency carrier after transmission of the transmission power control information TPC. Therefore, the transmission power control for the reverse link can be performed in the mobile station using the transmission power control information transmitted immediately before termination of the communication data in the base station. Therefore, degradation of quality of the reverse link can be successfully alleviated.

[0118] On the other hand, since the base station initiates transmission from the pilot signal PL, the mobile station can perform reception using the former and later pilot signal including the pilot signal transmitted initially upon starting of transmission of the communication data, upon reception of data immediately after the vacant period. Thus, reception quality of the forward line can be improved. Furthermore, the mobile station performs SIR measurement using the pilot signal PL immediately after vacant period, one greater number of transmission power control instruction can be transmitted to the base station to improve the transmission power control of the forward line.

[0119] Furthermore, as a timing of generation of the vacant period as the slotted mode, in addition to the method to generate the vacant period with a given internal as shown in step S3 of the flowchart in FIG. 2, a method to generate the vacant period depending upon a quality condition of the link by monitoring the link quality condition by the base station or the mobile station, can be considered. For example, the base station monitors link quality, such as frame error ratio of the signal from the mobile station, and issues notice for making the mobile station to measure the reception condition of other frequency carrier when the measured value exceeds the allowable value. Then, the base station enters into transmission mode including the vacant period. The mobile station is responsive to the notice to initiate control operation corresponding to the transmission mode.

[0120] On the other hand, similarly to the above, the mobile station monitors link quality, and issues a notice to the base station when the link quality degrades. Responding to the notice, the base station enters into the transmission mode including the vacant period. In conjunction therewith, the mobile station initiates control operation corresponding to the transmission mode. Furthermore, the base station monitors congestion condition of the link to generate the vacant period depending upon the congestion condition. As a method for entering into the transmission mode with the vacant period by monitoring the congestion condition of the link by the base station, the base station monitors number of mobile stations in communication with own station. When the number of the mobile stations is in excess of the predetermined value, notice is issued for a part of the mobile stations in communication for making them to perform measurement of the reception condition of other frequency carrier. Then, the base station enters into the transmission mode with providing the vacant period for the mobile station which issued the notice. Then, the mobile station receiving the notice initiates control operation corresponding to the transmission mode.

[0121] As set forth above, according to the present invention, in the communication system performing transmission with generating the vacant period in the slotted mode during data transmission in order to perform various process, such as different frequency carrier quality measurement or so forth, since the control signal for maintaining the communication quality can be inserted even in the vacant period, link quality may not be degraded even if the vacant period is long.

[0122] Namely, when the pilot signal is used as the control signal, even in the vacant period, measurement of the link quality of the forward link with the pilot signal is performed in the mobile station to report the result of measurement to the base station. Therefore, transmission power control of the forward link can be performed according to the report in the base station. Therefore, degradation of the forward link can be eliminated.

[0123] Also, when reception is performed using a plurality of pilot signals in the mobile station, degradation of the communication which can be caused by impossibility of use of the former and later pilot signals, can be successfully prevented.

[0124] By transmitting the transmission power control information of the reverse link in addition to the pilot signal as the control signal, the mobile station may perform control of the transmission power of the reverse link using the transmission power control information even in the vacant period. Therefore, degradation of the link quality of the reverse link can be prevented.

[0125] Although the present invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set out above but to include all possible embodiments which can be embodied within a scope encompassed and equivalents thereof with respect to the feature set out in the appended claims. 

What is claimed is:
 1. A mobile communication system comprising: transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames, and inserting a first control signal for maintaining a communication quality in said vacant period.
 2. A mobile communication system as set forth in claim 1, wherein said transmission control means inserts said first control signal at a predetermined time interval.
 3. A mobile communication system as set forth in claim 2, wherein said time interval is set to be longer than a time interval of said first control signal in a communication mode where transmission data are present in said communication frame.
 4. A mobile communication system as set forth in claim 1, wherein said transmission control means provides a vacant period from a timing immediately after a second control signal for maintaining communication quality.
 5. A mobile communication system comprising: transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames from a timing immediately after a second control signal for maintaining communication quality.
 6. A mobile communication system as set forth in claim 1, wherein said transmission control means transmits a third control signal for maintaining communication quality immediately after end of said vacant period.
 7. A mobile communication system as set forth in claim 1, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 8. A mobile communication system as set forth in claim 5, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 9. A mobile communication system as set forth in claim 6, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 10. A mobile communication system as set forth in claim 1, wherein said first control signal includes a transmission power control information for reverse link.
 11. A mobile communication system as set forth in claim 5, wherein said second control signal includes a transmission power control information for reverse link.
 12. A mobile communication system as set forth in claim 6, wherein said third control signal includes a transmission power control information for reverse link.
 13. A mobile communication system as set forth in claim 1, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 14. A mobile communication system as set forth in claim 5, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 15. A mobile communication system as set forth in claim 6, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 16. A mobile communication system comprising: transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of said vacant period, each of said second and third control signals being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 17. A mobile communication system comprising: transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of said vacant period, said second control signal being a transmission power control for a reverse link and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 18. A mobile communication system as set forth in claim 1, wherein said vacant period is provided by compressing a transmission data in communication in a time link.
 19. A mobile communication system as set forth in claim 1, wherein a communication mode is switched into a mode where said vacant period is provided at a predetermined time interval.
 20. A mobile communication system as set forth in claim 1, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said base station to said mobile station.
 21. A mobile communication system as set forth in claim 1, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said mobile station depending upon a link quality condition measured in said base station.
 22. A mobile communication system as set forth in claim 1, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said mobile station depending upon a congestion condition measured in said base station.
 23. A mobile communication system as set forth in claim 1, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said mobile station to said base station.
 24. A mobile communication system as set forth in claim 1, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said base station depending upon a link quality condition measured in said mobile station.
 25. A communication control method comprising: a step of providing a vacant period, in which no communication data is present, in one or more of communication frames, and a step of inserting a first control signal for maintaining a communication quality in said vacant period, for transmission.
 26. A communication control method as set forth in claim 25, wherein said first control signal is inserted at a predetermined time interval.
 27. A communication control method as set forth in claim 26, wherein said time interval is set to be longer than a time interval of said first control signal in a communication mode where transmission data are present in said communication frame.
 28. A communication control method as set forth in claim 25, wherein in said step of providing the vacant period, said vacant period is provided immediately after a second control signal for maintaining communication quality.
 29. A communication control method comprising: a step of transmitting a second control signal for maintaining a communication quality in one or more of communication frames; and a step of providing a vacant period, in which no communication data is present, at a timing immediately after said second control signal.
 30. A communication control method as set forth in claim 25, wherein a third control signal for maintaining communication quality, at a timing immediately after said vacant period.
 31. A communication control method as set forth in claim 25, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 32. A communication control method as set forth in claim 29, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 33. A communication control method as set forth in claim 30, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 34. A communication control method as set forth in claim 25, wherein said first control signal includes a transmission power control information for reverse link.
 35. A communication control method as set forth in claim 29, wherein said second control signal includes a transmission power control information for reverse link.
 36. A communication control method as set forth in claim 30, wherein said third control signal includes a transmission power control information for reverse link.
 37. A communication control method as set forth in claim 25, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 38. A communication control method as set forth in claim 29, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 39. A mobile communication system as set forth in claim 30, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 40. A communication control method comprising: a step of providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of said vacant period, each of said second and third control signals being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 41. A communication control method comprising: a step of providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of said vacant period, said second control signal being a transmission power control for a reverse link and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 42. A communication control method as set forth in claim 25, wherein said vacant period is provided by compressing a transmission data in communication in a time direction.
 43. A communication control method as set forth in claim 25, wherein a communication mode is switched into a mode where said vacant period is provided at a predetermined time interval.
 44. A communication control method as set forth in claim 25, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said base station to said mobile station.
 45. A communication control method as set forth in claim 25, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said mobile station depending upon a link quality condition measured in said base station.
 46. A communication control method as set forth in claim 25, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said mobile station depending upon a congestion condition measured in said base station.
 47. A communication control method as set forth in claim 25, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said mobile station to said base station.
 48. A communication control method as set forth in claim 25, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said base station depending upon a link quality condition measured in said mobile station.
 49. A base station in a mobile communication system, comprising: transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames, and inserting a first control signal for maintaining a communication quality in said vacant period.
 50. A base station as set forth in claim 49, wherein said transmission control means inserts said first control signal at a predetermined time interval.
 51. A base station as set forth in claim 50, wherein said time interval is set to be longer than a time interval of said first control signal.
 52. A base station as set forth in claim 49, wherein said transmission control means provides a vacant period from a timing immediately after a second control signal for maintaining communication quality.
 53. A base station in a mobile communication system, comprising: transmission control means for providing a vacant period, in which no communication data is present, in one or more of communication frames from a timing immediately after a second control signal for maintaining communication quality.
 54. A base station as set forth in claim 49, wherein said transmission control means transmits a third control signal for maintaining communication quality immediately after end of said vacant period.
 55. A base station as set forth in claim 49, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 56. A base station as set forth in claim 53, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 57. A base station as set forth in claim 54, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 58. A base station as set forth in claim 49, wherein said first control signal includes a transmission power control information for reverse link.
 59. A base station as set forth in claim 53, wherein said second control signal includes a transmission power control information for reverse link.
 60. A base station as set forth in claim 54, wherein said third control signal includes a transmission power control information for reverse link.
 61. A base station as set forth in claim 49, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 62. A base station as set forth in claim 53, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 63. A base station as set forth in claim 54, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 64. A base station in a mobile communication system, comprising: transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of said vacant period, each of said second and third control signals being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 65. A base station in a mobile communication system, comprising: transmission control means for providing a vacant period in which no communication data is present, in one or more of communication frames, from a timing immediately after a second control signal for maintaining a communication quality, and transmitting a third control signal for maintaining the communication quality immediately after end of said vacant period, said second control signal being a transmission power control for a reverse link and third control signal being a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 66. A base station as set forth in claim 49, wherein said vacant period is provided by compressing a transmission data in communication in a time direction.
 67. A base station as set forth in claim 49, wherein a communication mode is switched into a mode where said vacant period is provided at a predetermined time interval.
 68. A base station as set forth in claim 49, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said base station to said mobile station.
 69. A base station as set forth in claim 49, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said mobile station depending upon a link quality condition measured in said base station.
 70. A base station as set forth in claim 49, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said mobile station depending upon a congestion condition measured in said base station.
 71. A base station as set forth in claim 49, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said mobile station to said base station.
 72. A mobile station in a mobile communication system, comprising: quality measuring means for receiving a first control signal for maintaining a communication quality transmitted in a vacant period in which no communication data is present, in one or more of communication frames and measuring a reception quality on the basis of said first control signal; and transmitting means for generating and transmitting a transmission power control information for a forward link according to said reception quality.
 73. A mobile station in a mobile communication system, comprising: demodulation means for receiving a first control signal for maintaining a communication quality transmitted in a vacant period in which no communication data is present, in one or more of communication frames and demodulating a communication data using the first control signal.
 74. A mobile station in a mobile communication system, comprising: means for receiving a first control signal for maintaining a communication quality transmitted in a vacant period in which no communication data is present, in one or more of communication frames and controlling a transmission power in a reverse link according to said first control signal.
 75. A mobile station in a mobile communication system, comprising: quality measuring means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and measuring a reception quality on the basis of said second control signal; and transmitting means for generating and transmitting a transmission power control information for a forward link according to said reception quality.
 76. A mobile station in a mobile communication system, comprising: demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and demodulating a communication data using said second control signal.
 77. A mobile station in a mobile communication system, comprising: means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and controlling a transmission power in a reverse link on the basis of said second control signal.
 78. A mobile station in a mobile communication system, comprising: quality measuring means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after said vacant period, and measuring a reception quality on the basis of said second or third control signal; and transmitting means for generating and transmitting a transmission power control information for a forward link according to said reception quality.
 79. A mobile station in a mobile communication system, comprising: demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after said vacant period, and demodulating a communication data using said second or third control signal.
 80. A mobile station in a mobile communication system, comprising: quality measuring means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after said vacant period, and measuring a reception quality on the basis of said third control signal; transmitting means for generating and transmitting a transmission power control information for a forward link according to said reception quality; and means for controlling a transmission power in a reverse link on the basis of said second control signal.
 81. A mobile station in a mobile communication system, comprising: demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after said vacant period, and demodulating a communication data using said third control signal; and means for controlling a transmission power in a reverse link on the basis of said second control signal.
 82. A mobile station in a mobile communication system, comprising: demodulation means for receiving a second control signal for maintaining a communication quality transmitted at a timing immediately before a vacant period in which no communication data is present, in one or more of communication frames and receiving a third control signal for maintaining the communication quality transmitted at a timing immediately after said vacant period, and demodulating a communication data using said third control signal; quality measuring means for measuring a reception quality on the basis of said third control signal; transmitting means for generating and transmitting a transmission power control information in a forward link according to the reception quality; and means for controlling a transmission power in a reverse link on the basis of said second control signal.
 83. A mobile station as set forth in claim 72, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 84. A mobile station as set forth in claim 75, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 85. A mobile station as set forth in claim 78, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link.
 86. A mobile station as set forth in claim 72, wherein said first control signal includes a transmission power control information for reverse link.
 87. A mobile station as set forth in claim 75, wherein said second control signal includes a transmission power control information for reverse link.
 88. A mobile station as set forth in claim 72, wherein said first control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 89. A mobile station as set forth in claim 75, wherein said second control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 90. A mobile station as set forth in claim 78, wherein said third control signal includes a pilot signal to be used for demodulation of the communication data or a transmission power control for a forward link and a transmission power control information for reverse link.
 91. A mobile station as set forth in claim 72, wherein said vacant period is provided by compressing a transmission data in communication in a time direction.
 92. A mobile station as set forth in claim 72, wherein a communication mode is switched into a mode where said vacant period is provided at a predetermined time interval.
 93. A mobile station as set forth in claim 72, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said base station to said mobile station.
 94. A mobile station as set forth in claim 72, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice from said mobile station to said base station.
 95. A mobile station as set forth in claim 72, wherein a communication mode is switched into a mode where said vacant period is provided by issuing a notice to said base station depending upon a link quality condition measured in said mobile station. 